1. Technical Field
The present disclosure relates to sensors, and more specifically, to superconducting transition edge sensors and methods for design and manufacture thereof.
2. Introduction
A wide variety of particle detectors, energy detectors, and other devices can be made using a superconducting transition edge sensor (TES) as a thermal energy sensor, thermometer, bolometer, or microcalorimeter. By operating the device such that the TES is in the superconducting transition temperature region (i.e., the temperature region in which the material switches from normal conducting properties to superconducting properties) any heat deposited in the TES can be precisely measured due to the strong dependence of its conductivity (or conversely electrical resistance) on the temperature. Thus, very precise measurements of temperature changes and/or detection of an incident particles providing even minute heating, can be performed. The device can also be used as a high resolution spectrometer with the energy of the absorbed particle determined by the measured heat pulse in the detector (when used as a microcalorimeter) or a change in absorbed power when a flux of particles are detected (when used as a bolometer). Manifestation of devices include a single TES thermally connected to a single particle absorbing material, a single TES thermally connected to multiple absorbers, or multiple TESs thermally connected to a single absorber body, it is also possible for the TES sensor to act as both the thermistor and absorber. Considerations when selecting an absorber material and absorber design is chosen such that it efficiently absorbs the particle of interest, has a sufficiently fast thermalization time to meet timing requirements, is compatible connecting with the TES body, and a heat capacity large enough such that heat pulses from particles of highest energy do not saturate but no so large that the energy resolution is sufficiently degraded.
Several conventional methods for fabricating TES-based detectors are available. For example, a TES device with a body consisting of a uniform superconductor material, having a target superconducting transition temperature (Tc) corresponding to the temperature of interest can be fabricated. In another method, the superconducting materials can be doped with magnetic impurities. In such devices, the concentration of magnetic particles is used to modify the Tc of the superconducting material being used for the body. In another method, the body of the TES device can be formed using a bilayer structure consisting of a normal metal layer (i.e., a metal layer that is not superconducting at the operating temperatures of interest when in isolation) disposed on a superconductor layer formed on a supporting substrate. Such devices take advantage of the proximity effect. That is, when a clean interface is provided between a superconducting film and a normal metal film, and the films are thinner than their coherence lengths, the bilayer acts as a single superconducting film with a transition temperature suppressed from that of the bare superconductor. Consequently, the superconducting transition temperature (Tc) and the heat diffusion properties of the TES device can be modulated by careful selection of the thickness of the layers. However, precisely achieving a target Tc is typically difficult, principally due to the process variations typically associated with conventional TES device manufacturing processes.